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Management Of Diffuse Knapweed
Diffuse knapweed is a biennial or occasionally an annual or short-lived perennial that reproduces solely from seed. Similar to the other knapweeds, it tolerates dry conditions, however; diffuse knapweed can survive in higher moisture areas as well.
Mechanical Control
Pulling or digging is often feasible for control of scattered diffuse knapweed plants, either as new invaders or those persisting following herbicide treatments. While labor-intensive, it is effective if enough of the taproot is removed to discourage sprouting. A single "pulling event", even on an annual basis, is not enough to achieve control. Research indicates handpulling three times a year for as many years as additional knapweed appears may be effective. The first pulling is in spring when soil moisture allows extraction of a lethal portion of the taproot, followed by a second pulling in late June of bolting plants, and a third pulling just before seed dispersal to catch any plants overlooked previously. Plants from the last pulling should be burned, or otherwise carefully disposed of, to destroy viable seeds. Mowing is not a control method (because the taproot will not be removed) but can be used to reduce seed production (Roche and Roche 1999).
Chemical Control
Research conducted at Colorado State University indicates that dicamba (Banvel) at 0.5 to 1.0 lb. ai/A (0.5 to 1.0 qt) and picloram (Tordon) 0.25 to 0.5 lb. ai/A (0.5 to 1.0 qt) control diffuse knapweed. Tank-mixes of Banvel plus 2,4-D (0.5 + 1.0 lb. ai/A), Banvel plus Tordon (0.5 to 1.0 + 0.125 to 0.25 lb. ai/A), Tordon plus 2,4-D (0.188 + 1.0 lb. ai/A), clopyralid plus 2,4-D (Curtail; 1.5 + 8.0 oz ai/A) all control diffuse knapweed. Tank-mixes can save money and reduce grass injury resulting from higher use rates of a single herbicide (Beck 1999). These herbicides can be purchased at an agricultural products supplier (please check the yellow pages for specific stores).
Spotted knapweed and diffuse knapweed generally occupy the same areas in Colorado so the same herbicide treatments can be applied.
Biological Control
The Colorado biological control program on knapweeds utilizes a number of different insects that have been approved by the USDA. These include: Sphenoptera jugoslavica, a metallic wood boring beetle; Cyphocleonus achates, a root-boring weevil; Agapeta zoegana, a root boring moth; Metzneria paucipunctella, a seed feeding moth; Urophora affinis and U. quadrifasciata, two seed head flies; and Larinus minutus, a seed-feeding insect. By themselves, it is doubtful whether one organism would exert enough pressure on the plant population to effect any significant control. However, as a complex of biological agents, the effect should be enhanced.
Cultural Control
Long-term reductions of diffuse knapweed should include planting competitive plant species to occupy bare ground once infested by the weed. Many improved grass species can be seeded in late fall or winter when seedbeds are properly prepared. Also, maintaining fertile soils will assist in a healthy, weed-resistant plant population.
Resources
Beck, K.G. 1999. Diffuse and spotted knapweed. Colorado State Cooperative Extension. www.Colostate.edu/Depts/CoopExt
Roche, B.F. and C.T. Roche. 1999. Diffuse Knapweed. In: Biology and Management of Noxious Rangeland Weeds. Sheley, R.L. and J.K. Petroff. Oregon State University Press, Corvallis. pp. 217-230.